This doctoral thesis examines migration from a macro-demographic perspective, analysing its impact on population structure across various geographical scales, with a particular focus on international migration.
First, the demographic dimension is addressed through the theory of Demographic Metabolism, which describes social change via the succession of generations. Younger cohorts, with distinct characteristics, replace older generations; meanwhile, migration affects these groups unevenly in terms of age and generational size, thereby accentuating both intergenerational and intragenerational differences.
Second, this research further develops the notion of «fast demography», whereby migration can trigger rapid population changes, in contexts of economic booms, refugee inflows, or crises, especially when observed at regional or municipal scales. This shift is attributable to a context of very low fertility and mortality, in which migration becomes the central accelerating driver of demographic change.
The thesis is organized into five hypotheses comprising six articles. Two hypotheses (hypotheses 1 and 4) focus on the demographic dimension. On one hand, generations are delimited using both objective and subjective variables, highlighting the pivotal role of migration. On the other hand, Demographic Metabolism is operationalized through a multidimensional population projection at the regional scale, disaggregated by place of birth and educational attainment.
Other two hypotheses (hypotheses 2 and 3) address the geographical dimension of demographic acceleration. In the first instance, Population Turnover Rate is examined descriptively and the contribution of the components of demographic change at the provincial scale over the last two decades. In the second instance, turnover is related with local characteristics to inform public policy, first by analysing provincial demographic convergence through a regional vulnerability index, and subsequently by examining sub-municipal dynamics within the Barcelona Metropolitan Area.
The fifth hypothesis combines both concepts from a systemic perspective, studying age and cohort diversity at the sub-municipal level using three cities as case studies. This approach highlights the complexity of demographic shifts and contrasts with simplistic discourses on migration and conspiracy theories.
The findings indicate the consolidation of a Complex System of Demographic Reproduction across Spain, in which international migration acts as both a differentiating factor between generations and a core component of population growth. During the first migratory wave, prior to the Great Recession, international migration followed a more balanced territorial distribution. In contrast, during the second wave, its impact became regionally concentrated. This trend, combined with negative natural growth, has intensified territorial polarization in recent years, a dynamic that is expected to persist in the future.